Unnecessary bundle copy when building bundle container

Originally, the copying of bundles was tracked with a timestamp file, in combination with a CMake add_custom_command. However, attempts to resolve the problem with the add_custom_command proved unsuccessful, leading me to replace it with add_custom_target.

In my opinion, a custom target is less than ideal, as a custom target dependency always triggers whenever a target is built. A comparison of the time to run make -j when nothing has changed demonstrated that on my systems, it took twice as long (albeit still under 1 second).

_Originally posted by @pnoltes in https://github.com/apache/celix/pull/594#issue-1820929811